1. Field of the Invention
The present invention relates generally to integrated circuit fabrication and, more particularly, to a method and apparatus for measuring and calibrating the registration between overlying layers of a semiconductor wafer.
1. Description of the Related Art
The fabrication of complex semiconductor devices involves multiple processing steps. Multiple patterned layers of different materials are applied to a substrate to create the desired electronic semiconductor device. The different layers overlie each other and must be accurately registered to ensure proper operation of the semiconductor device. Displacement between corresponding features on different layers can degrade the performance of the device or can cause the device to be totally inoperative. As semiconductor devices have become increasingly complex, the feature dimensions have been correspondingly reduced. This reduction in feature dimensions has reduced acceptable tolerances on displacement between layers.
When a semiconductor device is fabricated, it is one of many chips on a wafer of semiconductor material. An example of a typical fabrication process is described below. First, a silicon wafer with a nitride is patterned and etched. A silicon dioxide layer is grown in the nitride windows, and the nitride is removed. Next, polysilicon is deposited on the wafer, aligned to the previous layer, and then the wafer is again patterned and etched. Then, the wafer is aligned to the previous layer, patterned, and diffused with dopants. After this step, a dielectric is deposited on the wafer, aligned to the previous layer, and then the wafer is again patterned and etched. Finally, a metal layer is deposited on the wafer, aligned to the previous layer, and then patterned and etched.
Higher circuit densities, smaller device sizes, and larger chip sizes have resulted from improved fabrication techniques. These advances require that the circuit patterns used for each step of the fabrication process be made more accurately so that they align with one another more precisely. To assist in the registration of overlying layers in semiconductor wafers, it has been common practice to include registration patterns or marks in each layer of the wafer. The patterns overlie each other and have a predetermined relationship when the layers are correctly registered. One commonly used registration pattern includes squares of different sizes on the layers to be registered. When the two layers are exactly registered, the squares are concentric. Any registration error produces a displacement of the squares relative to one another.
Since semiconductor wafers having multiple complex integrated circuits are expensive to fabricate, it is usually desirable to verify registration after the application of each layer to the wafer. If the displacement of the layers is outside tolerable limits, the defective layer can be removed and replaced with an accurately registered layer. Registration measurement, verification, and correction eliminate the scrapping of potentially good yielding wafers.
To avoid misregistration and its resulting expense, the registration must be accurately determined. In the past, it was common practice to verify registration manually. Experienced operators examined the registration of overlying patterns, e.g., typically pairs of bi-layer interleaved comb structures called optical veneers, on each wafer. However, such techniques were relatively slow and subject to human error and contamination of the semiconductor wafers.
More recently, automated systems for measuring registration have been developed. While these automated systems are vast improvements over the previous manual systems, a measurement system, nonetheless, unavoidably introduces certain errors into the measured values. The errors arise both in the optical and electronic portions of the automated systems. No known system can entirely eliminate or compensate for these errors, which are typically referred to as systematic errors.
In the past, it has been customary to calibrate such registration systems by comparing measurements with those obtained from another system, such as a scanning electron microscope (SEM), that is known to be accurate. However, such calibration techniques are relatively complex and require additional expensive equipment. Currently, the ability to calibrate registration measurement systems is limited to three techniques: (1) the indirect single layer design offset technique, (2) the SEM micrograph of multilayered structures technique, and (3) the tool induced shift (TIS) technique. All three techniques are limited in their applications and accuracy.
To describe the limitations of these techniques, the actual structure that is targeted for registration determination, and the conditions in which it is presented to the measurement system, must be described. The actual structure that is being used for registration determination includes an underlyinq substrate, a base layer, a developed photoresist layer, and, in most cases, an intermediary layer which is usually deposited for subsequent processing. An example of a typical measurement structure is formed as a box within a box so that registration measurement may be determined for the photoresist layer to the base layer. The registration measurement is determined by measuring the x and y components of the concentric boxes. The designed geometric center of the two boxes is defined as coordinate (0,0). The actual difference between the center of the inner box and the center of the outer box during wafer processing is the registration offset to be determined. A typical size for the inner box is 10 microns square. A typical size for the inner edge of the outer box is 20 microns square, with the outer edge of the outer box being 50 microns square. While these are not absolute numbers, they are submitted as typical sizes being employed in the industry.
In the single layer offset technique, both the outer and inner boxes are designed at the same layer, i.e., either the base layer or the photoresist layer. Offsets of the outer box relative to the inner box are designed into the structure. There is usually a group of structures with varying offsets in both the x and y directions. These structures are measured and the measured values are plotted against the design values. Equations of best fit are determined and used as calibration curves in the registration measurement system for measuring the actual multi-layer process topography. The limitation of this technique is that it does not represent the actual topography to be measured. Therefore, the calibration of the measurement system does not contain the optical characteristics of the structure to be measured. The most prominent missing optical factor is the optimum focus between the top of the photoresist and the top of the base layer. Other important factors include the refractive index and distortion of an intermediary layer.
The SEM micrograph technique is very difficult to use and limited in its applications and accuracies. This technique uses an SEM to take top-down micrographs of the multi-layered box-in-box structure or similar multi-layered registration structure. The micrographs are then used for measurements of the x and y components of registration. These measurements are determined through manual measurement with calipers and correlation to another SEM micrograph of a "known" standard. If the SEM system has direct measurement capabilities, then the taking and measuring of micrographs is not necessary. Once the SEM measurements of registration have been determined, they are plotted against the measurement system values. Equations of best fit are determined and are used as calibration curves for the registration measurement systems.
Since the SEM is considered to be the measurement standard of the semiconductor industry, one would believe that a calibration to its measurements would be the most accurate. However, there are some variables inherent to a SEM that limit both the applications and accuracy of this technique. First, the SEM is a highly sensitive system that relies on beam stability. Unfortunately, minor fluctuations in beam stability are commonplace and cause astigmatism in the resulting micrograph. Astigmatism results in error in the measurement of x and y registration. Further, it cannot be assumed that micrographs taken at different times have the same inherent beam characteristics.
Second, the SEM gains its notoriety through its ability to resolve very small features. It is assumed that this ability is transferable to the determination of very small differences in orientation. However, the SEM must be at very high magnification to resolve small feature differences. Unfortunately, the registration measurement structures currently employed are designed to be very large to maximize image field utilization and magnification of the registration measurement system, but this large size limits their imaging on an SEM to very low magnification. Therefore, the advantage of the SEM's high resolution is defeated through the use of low magnification.
Third, the SEM's output is the result of bouncing a beam of electrons off of a surface and tracing the paths of the scattered electrons to create an image. Unfortunately, the presence of an intermediary layer will block the beam from reaching the base layer. Therefore, all process levels that have an intermediary layer cannot be calibrated with this technique.
In the TIS technique, the registration measurement system (RMS) takes two measurements of a registration structure to determine the tool induced shift exhibited by the RMS. The first measurement is made with the registration structure at 0.degree. and the second measurement is made with the registration structure rotated by 180.degree. . The TIS calculated from these measurements can compensate subsequent measurements made with the RMS However, the TIS technique compensates for translational offsets only. It does not compensate for systematic magnification and distortion errors such as astigmatism.
The present invention is directed to overcoming, or at least minimizing, one or more of the problems set forth above.